Compounds are typically constituted by atoms bound together via chemical bonds. This is the case on a small-molecule scale (e.g. hexane or glucose) and also on a large-molecule scale (e.g. cellulose, single-stranded DNA, etc.). Since about half a century chemists have developed the tools to look at weak, flexible bonds, such as hydrogen bonds, that turn a single strand of DNA to the famous double helix. Such interactions are weaker, but are still between specific atoms, and many of them would still make for a strongly bound complex. But are there alternatives for such strong binding of two components?!

In chemistry this was unheard of 25 years ago, but think about two standard shoe laces that you can bind together via a knot: these are firmly bound to one another, but not via chemical bonds. Stoddart asked himself the question: would it be feasible to make molecules that are bound together not via strong or weak chemical bonds, but via the topology of the molecule, i.e. only via the spatial orientation of the atoms. It is rather easy to imagine such molecules (think about a molecule that looks like the 5 interlocked Olympic Rings), but yet this would be completely novel! Can we make molecular knots, or other similar constructs, and if so: how do they behave? Stoddart obtained the Nobel Prize last year for discoveries that started about 25 years ago: he developed new types of synthetic routes to construct these, and studied the properties thereof in detail. The three images below depict suchconstructs: The one on the left is a rotaxane, in which a molecule (here the green ring) is ‘stuck’ because of the big groups on either end of the blue molecule. In the molecule in the middle, a catenane, two rings are interlocked. The structure on the right is a plant-produced protein (a cyclotide), in which a short peptide chain (typically 30-35 amino acids) is structurally defined via three specific disulfide bonds that give the ring-shaped protein a specific, biologically active shape.

Figures: Materials that have properties causedby topological features caused by Mechanical Bonds.